The present invention relates to video clamp circuits and in particular to a circuit for compensating for residual clamp errors. The invention is useful in, but not limited to, the process of inserting a Vertical Interval Reference Signal, hereinafter referred to as VIRS, onto a particular line of a video program signal.
Modern television broadcasting standards require that the transmitted signal parameters such as video gain, sync amplitude, burst amplitude, chrominance/luminance gain ratio, setup level, and burst phase be maintained within specified limits. Some years ago, the visual test patterns transmitted during television station idle periods were deemed inadequate to meet the above modern requirements. Television broadcasters and networks began to transmit the above-mentioned VIRS along with normal television picture signals. The VIRS is transmitted while the channel is in actual operation.
Traditionally, the VIRS has been used to evaluate the transmission characteristics of a channel; for example to determine if the channel is meeting some predetermined quality standard. Thus, if a network program is transmitted over a telephone line, and the telephone company has contracted to provide distortion below a specified level, the VIRS gives an indication as to whether the requirement is being met. When undesirable distortion is found at the receiving end of a transmission channel, certain adjustments can be made at that end to compensate for distortions.
When the VIRS, or any other signal, is inserted on a program video signal it is desirable to closely match the blanking levels of the signals. This is accomplished conventionally by clamping the program video to a reference voltage which is equal to the blanking reference voltage of the VIRS. However, it has been found that a conventional clamp will not completely remove all of the commonly encountered low frequency distortion often present on program video signals. This distortion is visible on the television screen as streaks or lines.